Welcome to the World of Linux!
Ok, the problem you are having is that the X windows system does not know what the refresh rates of your monitor is. This information is vital because your screen can be damaged should it be wrong.
So what you need to do is contact the manufacturer or check your manual and look for the horizontal and vertical refresh rates of your monitor.
Next, you need to edit a file: /etc/X11/XF86Config and add in your refresh rates, there should be a template for this in the file.
After that, save the config, and $ startx
Voila! (Hopefully)